1. Field of the Invention
The present invention provides a structural component which may be used to control vibrations in many different situations. For example, the component may be a structural member of an aircraft, a part of a helicopter rotor or rotor blade, or part of a robotic arm.
2. Prior Art
There are many situations where it is desirable to control the vibrations of a component, particularly aeroelastic deformations arising in aircraft due to phenomena such as flutter, gust response and buffeting. Also, it is often desirable to reduce the noise generated by vibrating components, such as that produced by helicopter rotors and gas turbine blades. Further, it may be required to reduce vibrations in order for parts to be positioned accurately, as in the case of robotic arms where vibrations can interfere with their precise positioning.
The invention has particular application to helicopter rotor blades. The main source of noise and vibration in helicopters is aerodynamic. Noise is generated when the tip of the blade reaches transonic speeds and shock is produced. At present, the most developed active control technique to attenuate noise and vibration in helicopter rotors is that known as "higher harmonic control" or HHC. Typically, this involve the introduction of an harmonic frequency into the helicopter blade cyclic pitch control with a phase lag such that a redistribution of the aerodynamic loads is created. Another example of HHC is described in U.S. Pat. No. 5,314,308, which issued May 24, 1994 to, Reed, and in which each blade has a slotted cylinder which rotates at a speed which is a multiple of the rotor speed. The main problem with these systems is that the vibration frequency imposed on the blades is constant, whereas the blades of a helicopter experience airspeeds and vibrational effects which differ depending on their rotational position. Experiments with HHC have found that reduction of vibration by this technique may increase noise and vice versa.
Another approach to the helicopter blade problem has been so-called "individual blade control" or IBC. With this, each blade is controlled individually. This involves an active control that introduces an harmonic signal to achieve dynamic redistribution of forces. The signal can be introduced either by a pitch applied to the blade root, a flap located near to the blade tip, or local deformation of the airfoil. The actuators proposed in the past for IBC were heavy and unsatisfactory. Recently, however, there have been various proposals to use for this purpose so-called "smart materials", i.e materials having properties controllable by electric or magnetic fields. For example, U.S. Pat. No. 5,224,826, which issued Jul. 6,1993 to Hall et al., shows the use of a deflectable flap attached to a helicopter rotor blade which is moved by piezoelectric material, in order to control vibrations transmitted from the blade to the air frame.
There have also been suggestions for dynamically controlling the bending of a component such as a helicopter rotor blade by using a "smart material" embedded or incorporated in the blade. However, hitherto these smart materials have seemed to lack the energy required to work against the aerodynamic loads encountered. In order to generate significant changes in the aerodynamic spectrum, pitch deflections of at least two degrees are necessary. The maximum pitch deflection achieved so far is less than one half of this value.
The prior art also includes proposals for counteracting vibrations in other types of components by using smart materials. For example U.S. Pat. No. 5,141,391, which issued Aug. 25,1992 to Acton et al., refers to incorporating piezoelectric or magnetostrictive materials into gas turbine engine blades to provide dynamic damping. Here too it is found that the piezoelectric material usually does not develop sufficient energy to deal with the loads met in practice. In U.S. Pat. No. 5,382,134, it is proposed to use piezoelectric material to change the stiffness of a noise radiating element; however this only changes the natural frequency of the structure, and is not suited to IBC where rapid dynamic or cyclical control of vibration characteristics are required.
The problems which occur in applying smart materials to helicopter blades and gas turbine engines also occur in other fields, especially in aircraft where aeroelastic phenomena are concerned. In the aircraft field, these materials have hitherto not been successfully used to reduce the vibrations. This is because those smart materials, such as shaped memory alloys (SMA), that have the energy (here defined as the maximum stroke multiplied by the delivered force) to overcome the work done by typical aerodynamic loads encountered in flight, present poor dynamic response; conversely, those materials that have good dynamic response (such as the piezoelectric crystal PZT, the piezoelectric film PVDF, and the electrostrictive or magnetostrictive materials) lack sufficient energy. The latter types of smart materials have high stiffness and can deliver relatively large forces, but have only a very limited stroke, i.e. approximately 300.mu.-strain for PZT and electrostrictive materials, and 1,000.mu.-strain for magnetostrictive materials. Although the stroke may be amplified by mechanical means, a corresponding reduction in the force becomes an unavoidable trade-off.